Complaint initially gives Richard Osman anxious nights (5)
I believe the answer is:
groan
'complaint' is the definition.
(to groan is to complain about something)
'initially gives richard osman anxious nights' is the wordplay.
'initially' suggests taking the first letters.
The first letters of 'gives richard osman anxious nights' is 'GROAN'.
(Other definitions for groan that I've seen before include "Sound of one in pain" , "Titus -- (Mervyn Peake)" , "Sound of pain or despair" , "Make pained sound" , "See 18 Down" .)